MicroRNAs (miRNAs) are ≈22-nt noncoding RNAs that are processed from larger (≈80-nt) precursor hairpins by the RNase III enzyme Dicer into miRNA:miRNA* duplexes (1-3). One strand of these duplexes associates with the RNA-induced silencing complex (RISC), whereas the other is generally degraded (1). The miRNA-RISC complex targets messenger RNAs for translational repression or mRNA cleavage. There has been considerable debate about the total number of miRNAs that are encoded in the human genome. Initial estimates, relying mostly on evolutionary conservation, suggested there were up to 255 human miRNAs (4). More recent analyses have demonstrated there are numerous nonconserved human miRNAs and suggest this number may be significantly larger (5).
Both cloning and bioinformatic approaches have been used to identify miRNAs. Direct miRNA cloning strategies identified many of the initial miRNAs and demonstrated that miRNAs are found in many species (6-16). However, the throughput of this approach is low, and cloning approaches have appeared to approach saturation (8). Bioinformatic strategies have recently been used to identify potential miRNAs predicted on the basis of various sequence and structural characteristics (4, 7). However, such gene predictions may not point to all legitimate miRNAs, especially those that are not phylogenetically conserved, and all in silico predictions require independent experimental validation.
There is a continuing need in the art to identify additional miRNAs and to exploit their regulatory functions for human health.